Do Radiant Barriers Actually Work? Head to Head Comparison Test!
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- เผยแพร่เมื่อ 22 ก.ค. 2021
- There is a lot of debate online as to whether radiant barriers actually work, and if so - how. In this video, Jordan builds two identical doghouses using LP TechShield Radiant Barrier roof decking. The only difference in the two is that one dog house is built CORRECTLY - with the shiny side in towards the attic , and the other is built INCORRECTLY - with the shiny side facing out towards the sky. The results are impressive with the LP TechShield installed correctly beating the incorrect install by 18-20 degrees F. This translates directly into less heat load on cooling equipment and a lower energy bill, especially in hot climates.
So how does a radiant barrier work?
Heat transfers in three ways: radiation, conduction, and convection. The sun's rays radiate to the earth through space via, you guessed it, radiation, and require no medium like air, water, metal ect. to transfer. Some of the suns energy is radiated directly onto our homes roof where it is absorbed and transferred via conduction. Conduction is the transfer of heat through material. Think of a frying pan on a stove. The hot flame transfers heat into the frying pan, and the frying pan conducts through itself and into the food that is in direct contact. A cast iron frying pan's handle will get hot even though it has not come in direct contact with the flame. The heat has conducted through the pan and into the handle. The same thing happens on your roof. The suns radiant energy is absorbed into the roof and the heat is conducted through the roof materials to the bottom side of your roof deck. When the roof deck is hot, the energy will try to find equilibrium with the surroundings. Some of the energy will conduct into the surrounding rafters. Some of the heat will conduct into the adjacent air, which will in turn become buoyant and move, resulting in convection (the movement of heat through the movement of a medium) and some of the heat will re-radiate into the attic space heating the insulation below, which will in turn conduct heat towards your conditioned space.
Now, there are very effective strategies to combat the movement of heat from the roof to the conditioned living space. A well ventilated attic with allow heated air to escape through the ridge vent and allow cool air to enter through soffit vents limiting convection, and good insulation will limit conduction into the living space. But what about the third mode of heat transfer - radiation? This is where a radiant barrier comes into play. The aluminum backing on LP TechShield is a low emissivity material. This means that it is not a very good radiator of energy. So the energy that has been absorbed by the roof and conducted through the decking is not easily radiated off the underside of the roof decking and must find another way out - either by conduction, convection, or re-radiating back into the sky.
The real-world results for installing LP TechShield radiant barrier is about a 20 degree Fahrenheit cooler attic temperature, which is significant.
However, to achieve this advantage, it is critical that the product be installed correctly! This means that the low emissivity backing (the shiny side) is installed inward toward the attic. It seems intuitive to place to shiny side our to "reflect" the sun's rays, and this might have some effect if it weren't for the pesky need to put some sort of roofing in direct contact with the decking rendering the reflectivity pointless and changing the heat transfer mode to conduction.
Now that we understand how radiant barriers work, and have seen in real world testing that there is a significant difference in attic temperatures, the remaining question is that of cost. Is there an ROI for installing radiant barrier? In a renovation situation, I am not sure if a add-on product, whether foil or paint, is worth the time and expense. However, in a new build or complete re-roof of a ventilated attic where you must put down a roof deck anyway, I think it would be a missed opportunity to improving the comfort and performance of your home by using LP Techshield.
JUST MAKE SURE YOU INSTALL IT CORRECTLY!! SHINY SIDE IN!!!
Even the 'correct' one is incorrectly installed without an air gap.
This is wrong.
The property that aluminum has going for it is its emissivity coefficient, a measure of how much infrared it radiates compared to a perfect "black body" which has a coefficient of 1. Aluminum foil typically has a coefficient of 0.04. Wood is typically 0.9 or higher, so aluminum foil will radiate about 20 times less infrared than wood if they're both at the same temperature.
I wish more builders would get on that soap-box with you. That was the best explanation of radiation, convection and conduction I have ever heard! thanks.
this is great. now build one without any radiant barrier whatsoever and one with an air gapped radiant barrier
You should have made 3 dog houses. The third one should be made without a radiant barrier.
I don't have a dog.
Since no one is saying this you should have made 4 dog houses.
Easy way to remember it, is it needs an air gap to be able to work. Foil faces the air gap. I think min of 1 or 2" required.
Make a third doghouse and put the aluminum facing up. Tape the joints and nails. Put 2x4 furring on that. Deck it again then roof it. Check the difference between it and the others.
Your analysis of convection and conduction is correct, radiant is not. Heat does not travel by radiation, heat is the conversion of radiation into thermal energy.
Thanks for this video. What I did was do both. I just had my roof done and silver side up. I also had radiant barrier SPAYED done in my attic (giving me a 50-60 degree difference) and had 7-10 inches of insulation added, attic tent and I have two a/c units for each side of my 2500 sq. ft house. Sealed all my can lights, changed all light bulbs to LED's. My utilities went from $400 to $100. My thermostats are on 78 and 82. I also used purple insulation on the garage door. Whoo Hoo.
He returns!
thank you so much for clearly explaining how the 3 methods affect us and how Radiant barriers work. It's so great to see you putting out content again!!!
Great Explanation. I've been using radiant barrier and foam in all my new builds families are HAPPY this helps reduce Electric Bill here inTEXAS!
It's a great comparison, I would have liked to see the temperature difference inside the cabins as well. And I'd like to see if there would be a difference if you added venting in soffits and has a tower vent. Great stuff.
Thanks for that awesome explanation and sample.
what happens if you add an air gap between the shingle and the aluminum (faced up)? does the air gap change anything because now the shingle is not conducting heat?
Great video. Thanks very much for the info.
Excellent video. Thanks